The present invention relates to a magnetic resonance (MR) imaging system, and more particularly, to a radio frequency (RF) coil system for use within an MR imaging system.
A magnetic resonance (MR) imaging system provides an image of a patient or other object in an imaging volume based on detected radio frequency (RF) signals from precessing nuclear magnetic moments. A main magnet produces a static magnetic field, or B.sub.0 field, over the imaging volume. Similarly, gradient coils within the MR imaging system are employed to quickly switch into effect magnetic gradients along mutually orthogonal x,y,z coordinates in the static B.sub.0 field during selected portions of an MR imaging data acquisition cycle. Meanwhile, a radio frequency (RF) coil produces RF magnetic field pulses, referred to as a B.sub.1 field, perpendicular to the B.sub.0 field, within the imaging volume to excite the nuclei. The nuclei are thereby excited to precess about an axis at a resonant RF frequency. These nuclear spins produce a spatially-dependent RF response signal when proper readout magnetic field gradients are applied to them. The RF coil also is able to detect the RF response signal of the precessing nuclear spins and forwards the detected signal to the MR imaging system. The MR imaging system combines the detected RF response signals to provide an image of a portion of the body or object in the imaging volume.
In order to produce accurate images, the static B.sub.0 magnetic field, the magnetic field gradients and the B.sub.1 field generated by the RF coil need to be spatially homogeneous over the imaging volume. Traditionally, to produce homogeneous fields and gradients, the main magnet and gradient and RF coils have a cylindrical shape which completely surrounds the patient. In such systems, the B.sub.0 field is typically horizontal, running parallel to the longitudinal axis of the bore of the cylinder. The cylindrical shape and complete encasement of the patient insure a highly homogeneous imaging volume. The cylindrical configuration is disadvantageous, however, in that it severely limits access to the patient and the imaging volume. The cylindrical geometry makes it difficult, if not impossible, for a doctor to perform interactive procedures during an MR imaging scan. Additionally, many patients find the cylindrical bore of such traditional MR systems to be cramped, restricting the size of patients that can be examined and also causing claustrophobic reactions in some patients. Thus, alternatives to the traditional cylindrical geometry are needed.
In response to this need, open MR imaging systems have been developed. In an open MR system, the imaging volume is very accessible and open to both a patient and a doctor. This allows the access to the imaging volume for medical procedures, as well as alleviating the claustrophobic reaction of some patients. Some open MR systems utilize two disk-like magnet pole pieces positioned on opposite sides of the imaging volume with a vertical B.sub.0 field. These systems have gradient coils and RF coils that are also flat and disk-like in shape. These open MR systems provide a great amount of access for a doctor or patient in the space between the two disk-like magnet pole pieces. Other open MR systems use two toroid-shaped magnet pole pieces positioned on opposite sides of the imaging volume. When set up with a horizontal B.sub.0 magnetic field, the patient/doctor can access the imaging volume through the bore in the toroids or from the side. Since the magnet pole pieces are toroid-shaped, the corresponding gradient coils and RF coils are required to be similar in shape and flat to maximize the space between pole pieces. Thus, open MR systems alleviate the access and claustrophobia problems inherent with the traditional, closed system design.
Open MR systems are disadvantageous, however, in that it is more difficult to produce homogeneous magnetic fields within their imaging volume. In particular, the required flatness of the RF coil and other components in open MR systems. Similarly, because open MR systems do not completely surround a patient, it is difficult to obtain a high degree of homogeneity in the static B.sub.0 magnetic field, the gradient magnetic fields, and the B.sub.1 field.
One example of a typical open system RF coil is the dual butterfly design, which is especially inhomogeneous close to the conductors. A flat bird cage design in the form of a wheel and spoke structure may also be used, but it is also inhomogeneous near the conductors. As referred to herein, a system having a B.sub.1 field with high homogeneity has substantially equal sensitivity to RF signal throughout the imaging volume. When there is inhomogeneity in the B.sub.1 field, the sensitivity in the inhomogeneous area increases or decreases. This increase or decrease results in more or less RF signal being detected, resulting in bright spots or dark spots in the reconstructed MR image. So, for example, the area near the conductors in typical dual butterfly or flat bird cage designs are more sensitive than the rest of the imaging volume, thereby creating very bright areas or hot spots in the image.
Also, another disadvantage of typical flat RF coil designs is that the sensitivity does not drop off quickly enough outside of the imaging volume, resulting in RF fields outside of the imaging volume affecting the image. Inside the imaging volume, the B.sub.0 field, the gradient fields, and the RF field are designed to be as homogeneous as possible. Outside of the imaging volume, however, the homogeneity is not as controlled. As a result, in areas outside of the imaging volume, the superposition of inhomogeneous B.sub.0 and B.sub.1 fields and alinear gradient fields may give rise to aliasing of signal, where areas outside of the imaging volume generate a signal with the same frequency as areas inside the imaging volume. These outside signals may be detected and cause bright spots to be generated within the image. By sharply reducing the RF field sensitivity outside of the imaging volume, interaction of outside fields with the fields inside of the imaging volume is reduced. In typical prior art, the path of the return current is through the RF shield, in effect straight under the straight conductor. This results in a straight return current path along the center of the coil, where the return current path is not being used to produce a sharp drop off in sensitivity. As such, current RF coil designs typically result in stray RF fields outside of the imaging volume, the stray fields in combination with the non-linearity of the gradient coil and the inhomogeneity of the magnet can cause signal from far outside the imaging volume to fold into the image. Thus, in MR systems, it is desirable to produce a very sharp drop off in sensitivity outside of the imaging volume so that signal from outside of the imaging volume does not affect the image.
In addition to these disadvantages, the design of RF coils for open MRI systems have a number of other constraints. For example, the diameter of the RF coils is typically limited to the diameter of the magnet pole pieces. The diameter of flat RF coils, and their distance to the iso-center of the imaging volume, affects the ability of the coils to produce a homogeneous RF field. For example, it is easier to produce a homogeneous RF field inside an imaging volume when the diameter of the RF coils is equal to or greater than the distance from the RF coils to the iso-center of the imaging volume. As mentioned above, since the flatness of the RF coils already presents a homogeneity problem, restricting the allowable diameter of the RF coils adds another degree of difficulty to the problem.
Further, flat RF coils are inefficient compared to cylindrically-shaped coils that surround the imaging volume. Since flat RF coils are inefficient, they require a larger power amp than in a comparable closed MR system. A larger power amp is problematic because it adds additional cost to the system. Further, by requiring more power, the specific absorption ratio (SAR) of the RF fields generated by the RF coils may increase. As is known to those of skill in the art, SAR pertains to the level of electromagnetic energy which can be absorbed by a patient or medical personnel positioned in or close to the transmit RF coil of an MR system. Within the United States, for example, SAR limits are set by the Food and Drug Administration (FDA). Because of the tight spacing required for open MR systems, as mentioned above, the RF coils must necessarily be fairly close to the patient surface. Thus, the SAR limits can restrict the amount of power permitted to be utilized by the RF coils.